The present invention relates generally to voltage-to-frequency signal converters and more particularly to an improved voltage-to-frequency converter for generating periodic output signals that can be used for determining the frequency of operation of a time ratio control system that controls the magnitude of current supplied to an electric traction motor.
Large electrically driven traction vehicles such as locomotives or transit cars are propelled by a plurality of traction motors mechanically coupled to the respective wheel sets of the vehicle. Such motors are usually of the direct current (d-c) type. A d-c traction motor comprises a stator, a rotor, armature windings on the rotor, and field windings (either connected in series with the armature or separately excited) on the stator. In order to control its tractive effort, there is associated with the motor suitable means for regulating the magnitude of direct current in the motor armature. Electric power apparatus commonly known as a chopper is an energy conserving means for regulating armature current.
A chopper is essentially a controlled switch connected in circuit with the motor armature to meter current from a source of relatively constant d-c electric power to the motor. The switch is cyclically operated between open and closed states, and by appropriately controlling the timing of the successive transitions between these alternate states the magnitude of armature current can be varied or maintained substantially constant as desired. Assuming the chopper is in series with the motor and the propulsion system is operating in its motoring mode, during closed periods of the chopper the motor armature windings will be connected to the d-c power source through a path of negligible resistance, whereby virtually the full magnitude of the source voltage is applied to the motor armature and the current tends to increase. During the open periods of the chopper, the motor is disconnected from the power source and armature current, circulating through a free wheeling path, decays from the magnitude previously attained. In this manner, pulses of voltage are periodically applied to the motor, and an average magnitude of motor current (and hence torque) is established. The rate of change of current is limited by the circuit inductance.
The ratio of the closed time (t.sub.ON) of the chopper to the sum of the closed and open times (t.sub.ON +t.sub.OFF) during each cycle of operation is the duty factor of the chopper. For a 0.5 duty factor, the repetitive closed and open periods of the chopper are equal to each other, and the width of each voltage pulse has the same duration as the space between successive pulses. In practice, so long as the chopper frequency is relatively high (such as, for example, 300 Hz) the circuit inductance (including the inductance provided by the armature windings of the traction motor itself) will smooth the undulating current in the motor armature sufficiently to prevent untoward torque pulsations, whereby the vehicle is propelled without any uncomfortable amount of jerking or lurching. By varying the duty factor of the chopper, the average chopper output voltage (as a percentage of the d-c source voltage) and consequently the average magnitude of current can be increased or decreased as desired. This is popularly known as time ratio control or pulse control.
A propulsion system using choppers can be adapted for electrical braking by reconnecting the power circuits so that each chopper is connected to the d-c power source in parallel rather than in series with its associated motor. In the braking mode of operation, a traction motor behaves as a generator, and the magnitude of its generated voltage (electromotive force) is proportional to speed and field excitation. The excitation of a series field machine is a function of the magnitude of armature current. With the chopper reconnected in parallel with the motor, during its closed periods the chopper provides a low resistance path for armature current which therefore tends to increase, whereas during its open periods the armature current path includes the power source and the free wheeling path, whereby current tends to decrease. The electric power output of the motor is either fed back to the source (regenerated), or dissipated in a dynamic braking resistor grid that can be connected in parallel with the chopper, or a combination of both. In either case, the average magnitude of armature current (and hence braking effort) can be controlled as desired by varying the duty factor of the chopper.
In an electrically driven traction vehicle that is powered from a wayside source of electricity, appropriate filtering means will be included in the propulsion system of the vehicle so as to provide a desired degree of electrical isolation between the chopper and the wayside power conductors. When a plurality of chopper/motor units are connected in a parallel array to a common filtering means, the amplitude of ripple current in the filter could be undesirably and unnecessarily high if all of the choppers were operated in unison. Therefore it is good practice in a multiple unit propulsion system to stagger or "phase shift" the closed periods of the respective choppers so that they are sequentially initiated at substantially equally spaced intervals during each cycle of operation. This not only will reduce the amount of ripple that needs to be isolated from the wayside power conductors but also will minimize the rms current in the filter capacitor, thereby minimizing the size of this component.
In the present state of the art, choppers for traction vehicle applications use high-power, solid-state controllable switching devices known as thyristors or silicon controlled rectifiers (SCRs). A thyristor is typically a three-electrode device having an anode, a cathode, and a control or gate terminal. When its anode and cathode are externally connected in series with an electric power load and a source of forward anode voltage (i.e., anode potential is positive with respect to cathode), a thyristor will ordinarily block appreciable load current until a firing signal is applied to the control terminal, whereupon it switches from its blocking or "off" state to a conducting or "on" state in which the ohmic value of the anode-to-cathode resistance is very low. Once triggered in this manner and latched in by conducting load current of at least a predetermined minimum magnitude prior to removal of the firing signal, the thyristor can be turned off only by reducing the current through the device to zero and then applying a reverse voltage across the anode and cathode for a time period sufficient to allow the thyristor to regain its forward blocking ability. Such a device forms the main load-current-carrying switching element of the chopper, and suitable means is provided for periodically turning it on and off.
In practical applications the main thyristor of the chopper is periodically turned off by means of a "commutation" circuit connected in parallel therewith. A typical commutation circuit is a "ringing" circuit, i.e., the circuit contains inductive and capacitive components that develop an oscillating or ringing current. A chopper commutation circuit may include, for example, a precharged capacitor, an inductor, a diode, and the inverse parallel combination of another diode and an auxiliary thyristor. In a voltage turn-off type of chopper, these components of the commutation circuit are so interconnected and arranged as to divert load current from the main thyristor in response to turning on the auxiliary thyristor, and the main thyristor current is soon reduced to zero. The ringing action of the commutation circuit temporarily reverse biases the main thyristor which is consequently turned off, and during the reverse bias interval the current in the auxiliary thyristor oscillates to zero so that the latter component will also be turned off. For an ensuing brief interval, load current will continue to flow through the capacitor and a series diode in the commutation circuit of the chopper, thereby recharging the capacitor from the d-c source to complete the commutation process. Now the chopper is in an open or non-conducting state, and it cannot return to its closed or conducting state until the main thyristor is subsequently turned on by applying another firing signal.
The duty factor or percentage on time of the chopper is determined by the time delay between firing the auxiliary thyristor and subsequently firing the main thyristor during any full cycle of operation. The shorter this delay, the higher the duty factor, whereas the longer this delay, the lower the duty factor. Practical limits are imposed by the nature of the switching devices used in the chopper. For example, the maximum duty factor is approximately 0.91 for a chopper using a main thyristor rated 1,100 amps (average) and 2,000 volts (peak forward voltage) and operating at a constant frequency of approximately 300 Hz. A higher duty factor cannot be safely obtained at that chopping frequency because the aforementioned time delay must be at least 300 microseconds to make sure that the main thyristor is not refired prematurely, i.e., before the auxiliary thyristor has time to be completely turned off during the commutation process. For the same assumed parameters, the minimum duty factor would be approximately 0.09. This is because the minimum pulse width per cycle is determined by the recharging time of the capacitor in the oscillatory commutation circuit. Consequently, so long as it is being operated in a constant frequency variable pulse width mode, the chopper is effective to control motor current only in a limited range between its predetermined minimum and maximum duty factors.
It is generally desirable to be able to vary the chopper duty factor continuously over substantially the full range between 0 and 1.0. Smooth variations of the duty factor up to 1.0 are desirable during the motoring mode of operation to obtain maximum utilization of the available d-c source voltage when the vehicle is traveling at high speeds. Similar duty factor variations are also desirable during the braking mode of operation to obtain high, constant braking effort when the vehicle is traveling at low speeds. The higher the duty factor, the lower the minimum speed at which the maximum magnitude of armature current can be sustained during braking. Once the vehicle decelerates below this minimum speed, braking effort will decrease or fade out. In the improved propulsion system that is disclosed and claimed in co-pending U.S. patent application Ser. No. 35,352 filed concurrently herewith for R. B. Bailey and T. D. Stitt and assigned to the General Electric Company, an unusually low minimum brake fade out speed is obtained during electrical braking by operating the chopper in a constant high frequency, variable pulse width mode until the duty factor increases to its maximum at that frequency and then further increasing the duty factor by operating in a decreasing frequency, minimum "off time" mode.
For smooth motor starting it has heretofore been proposed to reduce the chopper frequency and operate in a variable frequency minimum pulse width mode so as to extend the range of duty factor variations below the minimum that can be obtained when the chopper is operated in a constant high frequency, variable pulse width mode. In a known method of this kind, a starting interval of predetermined duration is initiated by a motor starting signal, and during this interval the firing frequency of the auxiliary thyristor gradually increases from zero to the predetermined running frequency of the chopper. Firing of the main thyristor is temporarily inhibited during the starting interval, whereby current is supplied to the motor armature through the auxiliary thyristor. Due to the oscillatory nature of the commutation circuit in the chopper, each time the auxiliary thyristor is fired it conducts a narrow pulse of load current and is then automatically turned off. Therefore the average magnitude of the chopper output voltage will vary directly with chopping frequency during the starting interval.